The present invention relates to boilers in which water is heated by passing through a furnace and, in particular, it concerns features for minimizing hydrogen embrittlement and scale deposition in heat-exchange pipes of such boilers.
A number of factors limit the operational efficiency of modem steam-generating and hot-water boilers. Amongst the foremost of these factors are the problems of hydrogen embrittlement and of scale deposition.
Firstly, with respect to hydrogen embrittlement, hot water and steam generating pipes of steam-generating and hot-water boilers are conventionally made from expensive, heat-resistant high-carbon steels. The strong magnetic fields generated by flames within the furnace of the boiler result in magnetization of these pipes. Under the high temperature and pressure present within the pipes, partial dissociation of water molecules occurs and hydrogen atoms interact with the surfaces of the magnetized high-carbon pipes displacing the carbon atoms from the metal lattice and causing the effect known as hydrogen embrittlement of the heat-exchange pipes.
As a result primarily of hydrogen embrittlement, the life expectancy of heat-exchange pipes in steam-generating and hot-water boilers is usually no more than 8-10 years. During this period, cracks may develop in the most heat-stressed regions, resulting in down-time of the boiler. This possibility generally requires that an emergency team of repairmen be available at all times, and that frequent inspections are conducted to detect magnetized sections of pipe for their timely replacement.
A second major factor which complicates operation of boilers and lowers their efficiency is the constant formation of scale in heat-stressed regions of the heat-exchange pipes. Scale deposits greatly increase the thermal resistance of the pipes, thereby reducing the efficiency of heat exchange. The deposition of scale also necessitates regular washing out of the tubes, typically once or twice a month, as well as intermittent overhaul. Washing out of the tubes requires significant boiler down-time, thereby reducing profitability. Overhaul is a very expensive process which, for modern large-scale boilers weighing thousands of tons, may take several months.
There is therefore a pressing need for steam-generating and hot-water boilers with features for reducing hydrogen embrittlement and scale accumulation, thereby increasing the lifetime of the boiler and reducing the frequency of service interruptions in the boiler operation. It would also be advantageous to provide a method of operating a boiler to achieve similar goals.